In this commit we add a new endpoint so as to have a way of fetching
topic history for a given stream id without having to be logged in.
This can only happen if the said stream is web public otherwise we
just return an empty topics list. This endpoint is quite analogous
to get_topics_backend which is used by our main web app.
In this commit we also do a bit of duplication regarding the query
responsible for fetching all the topics from DB. Basically this
query is exactly the same as what we have in the
get_topic_history_for_stream function in actions.py. Basically
duplicating now is the right thing to do because this query is
really gonna change when we add another criteria for filtering
messages which is:
Only topics for messages which were sent during the period the
corresponding stream was web public should be returned.
Now when we will do this, the query will change and thus it won't
really be a code duplication!
Aditya Bansal